1. Field of the invention
The present invention relates to an automatic color adjustment apparatus for automatically changing only those colors in a specified area to another selected color while keeping the other colors of the image unchanged. This automatic color adjustment apparatus may be used in color printers, color photocopiers, color televisions, and other color image processing devices.
2. Description of the Prior Art
A variety of adjustments are required to obtain the required color control characteristics in conventional color image processing devices. These adjustments vary from such relatively simple adjustments as overall image luminance, color density, and RGB or CMY color balance control, to adjustments using image position data, such as color conversions applied to only a certain part of the image, and even more complex adjustments of the hue, chromaticity, or luminance of colors contained within a certain area.
The common objective of these adjustments is overcoming viewer dissatisfaction with the output image. The need for these adjustments is also commonly believed to drop as the performance of the color imaging devices improves and faithful color reproduction becomes possible.
It is important to note, however, that while the performance of the imaging device is one source of dissatisfaction with image quality, the subjective, psychological needs and desires of the viewer are an equally important factor. While "faithful color reproduction" is technologically possible, "desirable color reproduction" is subject to viewer preference as influenced by "remembered colors." Remembered colors are such things as skin color and green leaves, colors that the viewer remembers as being a certain color or that "should" be a certain color.
On video printers and other hard copy output devices it is more important for colors to be reproduced as the viewer believes they should be rather than being reproduced faithfully to the source image because it is the hard copy that will be kept. This is particularly true of remembered colors, and is even more true of skin colors. Faithful reproduction of skin color is often undesirable, and is a frequent reason why color adjustment of remembered colors is required.
Skin tones acceptable to the viewing audience are often reproduced in hard copy prints from television broadcasts recorded in a study because the recordings are made under bright lights and the actors appearing in the show are wearing make-up. The "remembered" skin colors are usually not reproduced in selected scenes from dramas, and even less frequently in amateur camcorder recordings. In the latter case, this is because make-up is not used, lighting is often too low and dependent on just available light, and the use of automatic white balance causes skin tones to be affected by background colors.
Conventional color adjustment used with television adjusts the chroma phase and level, and adjusts the luminance offset to adjust the colors when demodulating the NTSC signal to an RGB signal. Specifically, the hue is adjusted by changing the chroma phase, and the saturation is adjusted by changing the chroma level. In addition, changing the luminance offset also functions as a basic brightness adjustment. This adjustment method is both simple and very effective because it adjusts the color information, which has three attributes, using the three attributes most easily perceived by man: luminance, hue, and saturation.
Furthermore, a selective color adjustment apparatus which, while being physically large, allows the user to adjust colors in a selected area by converting the input signal to a color space defined by the three attributes of luminance, hue, and saturation, rotating the hue and adjusting the saturation of specific colors in this converted color space, and then reconverting the result to the original color space (cf., Gazou-Denshi-Gakkai-shi (The Journal of the Institute of Electronic Imaging Engineers) vol. 18, No. 5, pp. 302-312).
With these conventional color adjustment apparatuses, however, color adjustment applied specifically to remembered colors is difficult, and it is even more difficult to automatically adjust remembered colors.
An example is described below using skin color of Japanese as an example of remembered color. With the color adjustment methods used in television, hue adjustment is limited to simultaneous rotation of the color axis of all colors. Saturation and luminance adjustment are similarly limited to operations affecting the entire screen image. It is therefore not possible to adjust skin color alone without also affecting all other colors in the image.
The conventional selective color adjustment apparatus rotates the color axis and adjusts the saturation characteristic for a specific color area within the color space, and if the input color area that includes the skin color can be separated from other colors, skin color can be adjusted without affecting colors in the other areas. Automating this color adjustment process is virtually impossible, however, because determining which direction the hue axis should be rotated and how the saturation should be adjusted to obtain the "desirable" skin color depends upon the hue and saturation of the input skin color and subjective viewer preferences. As a result, user intervention is unavoidable.
The problem is further complicated by the inclusion of various skin colors in a single facial image, and it would be extremely rare that the luminance, hue, and saturation characteristics of all skin colors in the input image will need to be adjusted in the same direction and by the same amount. Because the direction and degree of adjustment desirable for the remembered skin colors is normally so variable, it is not possible for all skin colors in the input image to be corrected to the remembered color by the conventional selective color adjustment apparatus even if the area containing the skin colors can be specified.
As thus described, adjusting colors to the remembered color with conventional methods is extremely difficult manually, and is even more difficult to automate.